1. Field of the Invention
The present invention is related to systems for measuring and indicating the level and/or quantity of a fluid in a stationary container or storage tank and is particularly applicable to the problem of accurately measuring the level or amount of fuel in vehicular fuel tanks including fuel tanks used in aircraft, watercraft and landcraft. In a more directive sense, the invention relates to fluid level or fluid quantity measuring and indicating systems which include a capacitive type immersion sensor or probe.
2. Description of the Prior Art
Many techniques have been developed to measure the liquid levels and liquid quantities in storage tanks. The most common system for measuring the fuel level in the fuel tank of a motor vehicle employs a variable resistor within the tank. The wiper arm of the variable resistor is connected through a pivot to a float which monitors the upper level of the fuel in the tank. When the vehicle is traveling on a grade, the fuel surface level is shifted at an angle to its normal horizontal reference plane within the fuel tank and causes the float to monitor an erroneous level, either higher or lower than the correct level, and phenomenon termed "fuel level shift." Additionally, when the vehicle starts, accelerates, slows or stops its motion, waves are generated in the stored fuel. This phenomenon is commonly referred to as "sloshing" and causes the float to bounce up and down, thereby affecting the fuel measurement readings. Mechanical and electrical damping techniques have been employed to reduce the effects of fuel level shift and sloshing, but they have not been shown to be suitable for obtaining instantaneous and accurate fuel level measurements. The adverse affects of fuel level shift and sloshing are even more troublesome in the fuel tanks of aircraft and high speed landcraft and watercraft.
To overcome the problems of fuel level shift and sloshing in fuel level measuring and indicating, a number of systems have been proposed which use immersion condenser units or probes in liquid containers or tanks in combination with suitable electrical circuitry and measuring and indicating instrumentation. In U.S. Pat. No. 2,357,023, granted to O. W. Reid et al and entitled "Liquid Level Measuring Apparatus," the inventors disclose the use of a plate type immersion condenser unit located in a liquid container and for which the liquid itself forms the dielectric between the plates. The capacitive value of the immersion condenser is variable and changes with respect to the level of liquid in the container. By constructing the condenser unit of multiple pairs of plates which are placed in different areas of the container the effects of liquid sloshing or surging and liquid level shift are reduced so as to obtin a more accurate capacitance value for the immersion unit. The immersion condenser unit (whether of single pair or multiple pair plate design) is electrically connected into an alternating current bridge circuit having two condensers of fixed and equal capacity each forming an arm of the bridge, the remaining two arms of the bridge being made up of a variable capacity condenser and the immersion condenser. The bridge circuit (of well known Wheatstone configuration) is connected to a source of alternating current of predetermined frequency and detector and measurement indicating circuitry. The bridge circuitry is arranged to be in an unbalanced state so long as any liquid remains in the container in the dielectric space between the plates of the immersion condenser unit.
U.S. Reissue Patent No. 23,493, granted to A. Edelman and entitled "Liquid Level Gauge" also discloses liquid level detection and measurement indicating circuitry incorporating a plate type immersion condenser unit. This unit, termed a "measuring condenser," has a capacitive value which changes with respect to the level of liquid in a container or tank. The circuitry also includes a "comparison condenser" unit of plate type design which is always maintained fully immersed in the liquid to be measured as to its level or volume. The comparison condenser with respect to different liquids (having different dielectric values) is variable in its capacitance value but with respect to the measuring condenser, and its sensing of various levels of like liquid, provides a compensating or comparison capacitive value to the circuitry so that measurement of a liquid level is independent of the dielectric constant of the liquid and any variation thereof. Both the measuring condenser unit and the comparison condenser unit are exposed to substantially the same ambient conditions as the liquid being measured.
In U.S. Pat. No. 4,194,395, granted to T. J. Wood and entitled "Capacitive Liquid Level Sensor," a capacitive type sensor for measuring liquid levels is proposed in which a plurality of like plate type capacitors are aligned in parallel. The dielectric spaces of each capacitor (isolated from each other) receive the liquid to be measured which (with air, if any, above the liquid) establishes the dielectric for the spaces and thus the capacitance value of each capacitor. Since the capacitors are identically configured they exhibit equal values of capacitance only when the liquid dielectric (and air, if any) between the plates of each capacitor covers equal areas. When the liquid within a container is being measured as to its height level or volume and is sloshing or has its level disoriented with respect to its normal liquid level reference plane, the liquid (functioning as a dielectric) covers different areas of the capacitors and they exhibit dissimilar values of capacitance. Associated circuitry interrogates the capacitors and at points when the capacitance values approach equality the system reads one of the values and registers the liquid level or quantity of liquid remaining in the container.